This invention relates to finishing brake drums and more particularly to an apparatus using an abrasive film to micro-finish the braking surface of a brake drum.
It is generally required that brake drums, as typically found on motor vehicles, have their braking surfaces finished prior to mounting onto a motor vehicle. Previously, the finishing of brake drums has been of two varieties.
One variety finishes the braking surface by drawing a single point tool across the braking surface as the brake drum is rotated. However, as the single point tool is drawn axially across the braking surface, the tool inevitably carves a helix into the braking surface. During actual use of a brake drum finished in this manner, the helix will operate as a "screw" when a brake shoe is applied against the braking surface and will axially draw the brake shoe along the helix generally out of the brake drum. When the applied pressure of the brake shoe is released, the mechanical linkages of the brake shoe abruptly pull the brake shoe back into its proper position. Often, a result of the brake shoe being brought back into its proper position is that the brake shoe strikes against other components of the brake system and induces unwanted mechanical shock, vibration and noise, also known as "brake slap". Additionally, the use and often replacement of single point tools is costly.
Another method used for finishing the braking surface substitutes a honing stone for the single point tool. The results, however, are not a substantial improvement. During the finishing of the braking surface, the honing stone is drawn across the rotating braking surface. The lead angle of the honing stone similarly causes a helical groove to be cut into the braking surface of the brake drum. The helix again operates to pull an applied brake shoe axially out of the brake drum which results in mechanical shock, vibration and brake slap once the brake shoe is released.
An additional aspect of the brake drum having a helix in the finish of its braking surface is that the surface roughness (R.sub.a) of the finish is greater than that required for braking purposes. A smoother braking surface would increase the wear life of the drum lining and the braking material of the brake shoe.
Another limitation occurring in brake drums is a departure of the braking surface from true roundness (CG). This is partially a result of the finishing process not being utilized to geometrically alter the shape of the braking surface, but rather, solely being used to polish the braking surface. Thus, the degree to which the braking surface is cast "out-of-round" will not be corrected by the finishing process.
A further limitation often found in brake drums is that the braking surface, when viewed in cross section, is not straight and also exhibits a bore taper from the inboard portion of the braking surface, when compared against a reference axis drawn through the center rotation of the brake drum. As bore taper increases and straightness decreases, the brake shoe experiences twisting or only partial application against the braking surface. Preferably, the braking surface is straight and exhibits little or no taper.
Eccentricity of the braking surface, relative to the center of rotation of the brake drum, is also undesirable and results in uneven finishing of the braking surface and uneven wear of the brake pad and drum lining. During initial formation of a composite brake drum, eccentricity can result from a separate mounting plate being incorporated into a casting mold and the casting of the remaining portions of the brake drum thereonto.
With the above and other limitations in mind, it is an object of the present invention to provide a micro-finishing apparatus and method for micro-finishing the braking surface of a brake drum without the production of a helix in the finish of the braking surface. In this manner, the present invention seeks to eliminate the brake slap, vibration and mechanical shock which typically accompany the formation of a helix in the braking surface. By eliminating the helix, the present invention also seeks to decrease the surface roughness previously associated with brake drums.
An additional object of this invention is to permit the micro-finishing process to affect the geometry of the braking surface. In this manner the present invention aims to increase the roundness of the braking surface, increase its cross sectional straightness, and to decrease the bore taper of the braking surface.
Another object of the present invention is to decrease eccentricity existing between the braking surface and the center rotation of the brake drum itself. In achieving reduced eccentricity, the apparatus and method of the present invention seeks to gage the micro-finishing of the braking surface from a center rotation established by the braking surface.
It is also an object of this invention to simulate, during micro-finishing, the actual forces experienced by the brake drum when mounted onto a motor vehicle for use.
Still another object of this invention is to decrease the costs associated with finishing the brake drum braking surface.
In achieving the above objects, the present invention provides a micro-finishing apparatus and method for micro-finishing the inner cylindrical braking surface of a brake drum. A center of rotation is gaged from the braking surface and is used to establish the actual center of rotation for the brake drum. Thus eccentricity between the two is eliminated. The brake drum is mounted to the micro-finishing apparatus in a manner which simulates the forces experienced by the brake drum when actually mounted to a motor vehicle. The micro-finishing apparatus uses an arcuate tool or shoe having a surface which generally corresponds to the cylindrical braking surface of the brake drum. An abrasive film is supplied so as to be positioned over the surface of the shoe. During micro-finishing, relative movement is established between the abrasive film and the braking surface with the abrasive film and the shoe being urged against the braking surface of the brake drum. The abrasive film can also be advanced during the actual micro-finishing process to continually provide for the use of new or clean film. The apparatus is also provided with an assembly for oscillating the abrasive film relative to the braking surface.
In using the apparatus of the present invention, the micro-finishing process can be outlined as follows:
a) gaging a center of rotation for the brake drum off of the braking surface of the brake drum;
b) forming a mounting bore in the brake drum which is concentric with the center of rotation established by the braking surface;
c) locating the mounting bore of the brake drum on the micro-finishing apparatus so that a rotational axis of the micro-finishing apparatus will correspond with the center of rotation established by the braking surface;
d) securing the brake drum on the micro-finishing apparatus in a manner simulating the mounting forces experienced by the brake drum during actual use;
e) rotating the brake drum;
f) positioning an abrasive film over a surface of a finishing tool; and
g) urging the abrasive film into contact with the braking surface to micro-finish the brake drum braking surface.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims taken in conjunction with the accompanying drawings.